1. Field of the Invention
The invention is directed to a method of chemically modifying a cellulosic hydrogel as a sorptive support material such as a cellulose bead. More particularly, chemical modifications are done on solvent exchanged cellulose beads in a stringently non-aqueous environment.
2. Description of the Prior Art
Hydrogels are a class of organic substances capable of retaining and "immobilizing" water in a gel state having low solids content. Hydrogels usually consist of large organic macromolecules, and are hydroxy functional polymers capable of supporting a porous network structure. Porous hydrogels are useful materials in conjunction with sorption, flow, and separation processes involving aqueous solutions. Although hydrogels can be produced in various shapes and forms, such as films, fibers, and spherical "beads", they are most commonly used in bead form because of the ability to pack beads into continuous flow columns. Beaded hydrogels from such water-insoluble polysaccharides as dextrans and cellulose have gained practical importance in the separations industry.
Cellulosic hydrogel beads have particularly attractive characteristics in terms of flow, strength, and pore (or network) dimensions. Polysaccharide gels are often materials with less than 25% solids content when drained of all free water; and solids content may be as low as 2% in the case of cellulosic beads. Hydrogels from polysaccharides are generated from solution state by crosslinking or precipitation in a non-solvent following such a shape formation procedure as atomization. Polysaccharidic hydrogels have excellent properties in terms of crush resistance when exposed to flowing streams of water. Owing to their network structure and/or porosity, they are capable of accommodating large macromolecules (e.g., MWs of 5-500,000) such as proteins and enzymes, in their pore structure.
Shaped hydrogels from polysaccharides are limited by the fact that they lack specific bonding sites capable of separating and distinguishing different water-soluble molecules on grounds of specific functionality. This apparent inactivity is usually overcome by the reaction of hydrogels in aqueous medium, usually in aqueous alkali, and in reactions usually producing ethers.
Activation/functionalization of hydrogels from polysaccharides involving aqueous alkali represent state-of-the-art hydrogel modification techniques. Although this procedure prevents the complete desiccation of the solids leading to the formation of an amorphous glassy structure, this type of modification results in dimensional changes that alter the porosity and network dimensions of the activated gel structure. Not only is the chemical modification of pre-shaped hydrogels limited to aqueous medium, this also results in loss of porosity.
No method exists today, or has been described in the state-of-art, that permits the modification of a pre-shaped hydrogel in non-aqueous medium without loss of gel structure.